1. Field of the Invention
The present invention generally relates to the polymerization of a self-assembled monolayer (SAM) that contains conjugated π bonds. In particular, the present invention relates to disposing bifunctional molecules on a substrate to form a SAM, where the bifunctional molecules include an end-cap functionality, which is polymerized to form a SAM that is interconnected through conjugated π orbitals to form a charge transport pathway. More particularly, the present invention relates to the fabrication of an organic field-effect transistor, in which the channel material comprises a SAM, which forms an intramolecular charge transport pathway across the channel of the organic field-effect transistor by a conjugated polymer strand.
2. Description of Related Art
Conventional organic field-effect transistors (O-FETs) are based on thin films of organic semiconductors that are deposited on a substrate between two electrodes, i.e., a source and a drain, either by vacuum sublimation or deposition from a solution or a melt. These conventional O-FETs require a large number of molecules to bridge the gap between the source and the drain electrode. In these conventional O-FETs, conduction of the charges through the large number of molecules is facilitated by the large intermolecular overlap between π-electron orbitals of nearest neighbor molecules.
However, even in the best case, where organic molecules are highly aligned and such overlap is maximized, intermolecular charge transport between molecules is hindered by phonon scattering caused by molecular vibrations, which are substantial even at room temperature. Unless the organic molecules of an O-FET are cooled to very low temperatures, where molecular vibrations are reduced significantly or practically eliminated, the charge mobility of organic semiconductors is limited to values on the order of 1 cm2 V−1 s−1. In these conventional O-FETs, the mobility of charges depends on the intermolecular π-electron overlap between adjacent organic molecules and the weak Van der Waals forces acting between the molecules.
On the other hand, if charge transport occurs via π-electrons of a conjugated molecule, i.e., either a small organic molecule or a single polymer strand, then the charge transport is not restricted by the intermolecular charge transport mechanism described above. To make intramolecular charge transport via π-electrons of a conjugated molecule workable for both very small, i.e., a few nanometers, and for large area O-FETs, the length of the organic semiconductor molecule must be at least equal to the length of the field-effect transistor channel. Since most organic molecules are short, i.e., about 1 to 5 nm in length, relative to the minimum field-effect transistor channel length achievable with existing photolithographic techniques, it is extremely difficult to fabricate O-FETs, whose operation is based on an intramolecular charge transport mechanism, i.e., the transport of charges by the π-electrons of an organic molecule having conjugated bonds.
Polymerization of SAMs, especially the synthesis of a polymer brush, is known in the art. For example, the photopolymerization of acrylonitrile SAMs on the surface of a gold substrate to form a polymer brush has recently been described. Similarly, the polymerization of a SAM containing a styrene end group to form polystyrene brushes has also been described. In these examples, a bifunctional molecule, including an end-cap functionality, is self-assembled on a substrate and the end-cap functionality is polymerized through various means, such as, electrochemical, free radical, or photochemical polymerization. However, these resulting polymers do not contain any π-orbital conjugated bonds, which are necessary for the intramolecular transport of charges.
The polymerization of SAMs comprising pyrrole or thiophene end-cap functionalities, where the end-cap functionalities are used as nucleation sites for the growth of conjugated polymers, is also described in the art. For example, the electrochemical polymerization of pyrrole of a SAM with a pyrrole end-cap functionality, as a means of epitaxial growth of poly(pyrrole) on the SAM's surface, has been described. In these examples, the pyrrole end-cap functionalities of the SAM are used as nucleation sites for polymerization of external pyrrole. Although a conjugated polymer is the end result of this polymerization, this form of polymeric monolayer does not constitute an intramolecular charge transport pathway, which is located parallel to the surface of a substrate.
SAMs have been polymerized to form conjugated π-orbitals, where bifunctional molecules, containing a chlorosilane functionality at one end and a thiophene, a bithiophene, or a terthiophen end-cap functionality on the other end, were self-assembled on a substrate and then polymerized to form polythiophenes, comprising conjugated π-orbital charge transport pathways parallel to the surface of a substrate. However, although the formation of this polymeric SAM was demonstrated by UV-visible spectroscopy and cyclic voltametry, the charge transport properties, e.g., charge mobility, of this polymeric SAM were not measured and no functional electronic device, in which the polymer formed an active element of an electronic device, was constructed. Moreover, no electronic device has been constructed in which charge transport occurred by intramolecular charge transport pathways.